Apparatus for measuring microvascular blood flow

ABSTRACT

An apparatus for measuring microvascular blood flow in tissue including a monochromatic light source arranged to irradiate a section of the tissue with the monochromatic light from the light source, a photodetector arranged to collect light scattered from the irradiated section, a processor for processing the electrical output signals from the photodetector, calculating the power spectrum of photocurrents generated in the detection of laser light scattered from static tissue and Doppler broadend laser light scattered from moving blood cells, and recording the average Doppler frequency shift, and further calculating and recording the blood concentration. The apparatus further measures and records the intensity of the detected scattered light, calculates and records the blood perfusion (flux), filters movement artefact noise, and displays the blood perfusion measured parameters. By filtering movement artefact noise, the apparatus enables fast tissue blood perfusion monitoring with enhanced signal quality.

The present invention relates to an apparatus for measuringmicrovascular blood flow.

Blood flow in the small blood vessels of the skin performs an essentialrole in the regulation of the metabolic, hemodynamic and thermal stateof an individual and the condition of the microcirculation over bothlong and short time periods can reflect the general state of health. Thedegree of blood perfusion in the cutaneous microvascular structure oftenprovides a good indicator of peripheral vascular disease and reductionof blood flow in the microcirculatory blood vessels can often beattributed to cutaneous vascularisation disorders; so there are manysituations in routine clinical medicine where measurement of the bloodflow is important.

The microcirculation, its responses to stimuli, and its response totherapeutic regimes, were not open to routine continuous assessment andinvestigation until the introduction of the laser Doppler technique inthe 1970's and subsequent developments in the 1980's.

The technique depends on the Doppler principle whereby laser light(which must be highly monochromatic and hence have a long coherencelength) which is incident on tissue (typically the skin surface), isscattered by moving red blood cells and undergoes frequency broadening.The frequency broadened laser light, together with laser light scatteredfrom static tissue, is photo detected and the resulting photo currentprocessed to provide a signal which correlates with blood flow.

Laser light can be directed to the tissue surface either via an opticfibre or as a light beam. For “fibre optic” monitors the optic fibreterminates in an optic probe which can be attached to the tissuesurface. One or more light collecting fibres also terminate in the probehead and these fibres transmit a proportion of the scattered light to aphoto detector and the signal processing electronics. Normal fibreseparations in the probe head are a few tenths of a millimeter soconsequently blood flow is measured in a tissue volume of typically 1mm³ or smaller.

When a larger volume of tissue is stimulated to vasodilate orvasoconstrict, or where for example a healing process results inincreased blood flow, the measured blood flow changes in the smalltissue volume is generally taken to be representative of the largervolume.

For laser beam monitors single point measurements can be made bydirecting the beam to the desired point on the surface. By scanning thebeam in a raster fashion a series of measurements can also be made, andby colour coding the flow measurements a colour image of blood flowdistribution over the scanned surface can be displayed on a computermonitor screen.

Single point measurements give a high temporal resolution (40 Hz datarates are typical) enabling rapid blood flow changes to be recorded,whereas the laser Doppler imager can provide spatial information and hasthe ability to average blood flow measurements over large areas. Fibreoptic systems can measure at tissue sites not easily accessible to alaser beam. For example measurements in brain tissue, mouth, gut, colon,muscle and bone.

Perfusion measurements using single and multiple channel fibre opticlaser Doppler monitors have been made on practically all tissues andapplied in most branches of medicine and physiology. The technique andits application has been described in numerous publications. Arepresentative selection of these are included in “Laser-Doppler BloodFlowmetry”, ed. A. P. Shepherd and P.Å. Oberg, Kluwer AcademicPublishers 1990 and also “Laser Doppler”, ed. G. V. Belcaro, U.Hoffmann, A. Bollinger and A. N. Nicolaides, Med-Orion Publishing Co.1994.

The basic principles of measuring blood flow using coherent radiationand the Doppler effect were first described by C. Johnson in U.S. Pat.No. 3,511,227 patented May 12, 1970 entitled “Measurement of Blood Flowusing Coherent Radiation and Doppler Effect”.

The application of these principles to measurements in themicrocirculation was described by M. D. Stern in “Nature”, Vol 254, 56,March 1975, “In vivo evaluation of microcirculation by coherent lightscattering”; M. D. Stern et al, 1977 “Continuous measurement of tissueblood flow by laser-Doppler spectroscopy”, Am J. Physiol 232: H441-H448;and subsequent in U.S. Pat. No. 4,109,647, Aug. 29, 1978 “Method of andapparatus for Measurement of Blood Flow using Coherent Light”.

An apparatus using fibre optics to transmit the laser light to tissuesite and collect scattered light using one or more optic fibres wasdescribed by Holloway, G. A. and D. W. Watkins, 1977, “Laser Dopplermeasurement of cutaneous blood flow”, J. Invest. Dermatology 69: 306-309and D. W. Watkins and G. A Holloway, 1978, “An instrument to measurecutaneous blood flow using the Doppler shift of laser light”, IEEE TransBiomed Eng BME-25: 28-33. Extensions to theory and investigation ofexperimental models were made by R. Bonner and R. Nossal June 1981, Vol20 No. 12, Applied Optics, “Model for laser Doppler measurements ofblood flow in tissue”. They showed that the first moment of the powerspectral density of the photo current produced by the heterodyne mixingof Doppler shifted and unshifted laser light scattered from themicrovasculature could be used as a measurement of perfusion. Thisparameter is commonly referred to as “Flux”. They described the photoncharacteristics both in terms of auto correlation functions and spectralproperties and used photo correlation techniques for their experimentalinvestigations.

A perfusion monitor based on the application of auto correlationtechniques is described by R. J. Adrian and J. A. Burgos “Laser Dopplerflow monitor”, U.S. Pat. No. 4,596,254, Jun. 24, 1986.

In the present investigation we have used mainly digital signalprocessing but have chosen to use the technique of Fast FourierTransformation, implemented with large scale digital signal processor(DSP) ICs, for the Flux calculations. This enables the high data ratesnecessary for real time graphical display.

The algorithms we have implemented have the important advantage thatnoise due to fibre movements, a major problem in existing laser Dopplerfiber optic instruments, is generally reduced to insignificant levels.Using FFT processing with post DSP systems has additional advantages inthat processing algorithms can be changed without a corresponding changein hardware. For example, the processing bandwidth for the Dopplershifts can be changed, measurements at different bandwidths can be donesimultaneously; different frequency weighting in the “flux” calculationcan be used to provide a means of easily differentiating fast from slowblood flows and hence provide a means of depth discrimination.

Reducing the use of analogue circuits to a minimum has the addedadvantages of greater reliability, reduced size and weight, and reducedmanufacturing and servicing costs.

The present invention provides an apparatus for measuring blood intissue comprising:

a monochromatic light source;

means for irradiating a section of the tissue with the monochromaticlight from the light source;

means for collecting light scattered from the irradiated section;

means for photodetecting the collected scattered light;

means for processing the electrical output signals from thephotodetector;

means for calculating the power spectrum of the photocurrents generatedin the detection of laser light scattered from static tissue and Dopplerbroadened laser light scattered from moving blood cells;

means for calculating and recording the average Doppler frequency shift;

means for calculating and recording the blood concentration;

means for measuring and recording the intensity of the detectedscattered light;

means for calculating and recording the blood perfusion (flux);

means for filtering movement artefact noise;

means for displaying the blood perfusion measured parameters.

Embodiments of the invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 is a block diagram of an apparatus for laser Doppler perfusionmonitoring, said apparatus including a DSP for real-time fluxcalculation and removal of the fibre movement artefact in accordancewith the invention;

FIG. 2 is a flowchart of real-time FFT and flow calculation using DSP inaccordance with the invention;

FIG. 3 is a flowchart of real-time FFT and flow calculation using DSPfor a multichannel system in accordance with the invention;

FIG. 4 shows the power spectra obtained from skin with and without fibremovement;

FIGS. 5-6 are two examples of blood flow outputs with and without fibremovement artefact removed using the apparatus of the present invention;

As shown in FIG. 1, red or near infra-red light from a low power laser(2) is directed via an optical fibre (1) to the tissue and the lightscattered back from the tissue is collected by one or more other opticalfibres (1) and received by the photodetector (2). The photodetectorconverts the optical signal into an electrical signal. A bandpass filter(3) is used to remove noise outside the bandwidth and extract blood flowrelated AC components. A low-pass filter (4) is also connected to theoutput of the photodetector and is used to extract DC componentsproportional to the intensity of the collected light. Outputs of thebandpass (3) and low-pass filter (4) are converted into digital form bya multiplexer and A/D (5). Spectral analysis of the digitised Dopplersignal, blood flow calculation and movement artefact detection andremoval are performed by the powerful DSP device (6) in real-time.

FLOW CALCULATION

Laser light reflected and scattered from tissue consists of twofractions, one which is unchanged in frequency and one which has aDoppler broadened fraction due to interactions with moving blood cellsin the microvasculature of the tissue. The performance of any laserDoppler flow monitor (LDF) mainly depends on the nature of the signalprocessing algorithm and the way of implementing the algorithm. Sincethe introduction of the first LDF monitor, many different methods ofobtaining a reliable blood flow measurement have been proposed in theliterature. Frequency weighting the detected signal, which essentiallyintroduces a velocity-dependent multiplier into the signal processing,has become the most frequently used method for blood flow monitoring.This algorithm can be expressed by:ω  weighting:Flux=∫_(ω₁)^(ω₂)ω  P(ω)ω

Other ω weightings can also be used. For example, an ω² weighting willgive increased weighting to scattering from fast moving red blood cells.The algorithm is: ω²  weighting:Flux=∫_(ω₁)^(ω₂)ω²  P(ω)ω

where ω₁ and ω₂ are lower and upper cut-off frequencies of the bandpassfilter, P(ω) is the power spectral density.

Because of the complicated and time consuming computation of a largenumber of power spectra, most LDFs adopt an analogue approach toimplement the above processing, through Adrian et al (U.S. Pat. No.4,596,254, Jun. 24, 1986) describe a digital processing technique whichemploys a simplified autocorrelation algorithm to achieve continuous andreal-time computation of blood flow.

The recently available DSP devices can perform 1024 points FFTcalculations within 10 ms, which makes it possible to compute flowoutput directly in frequency spectrum form as described in the ω and ω²weighted algorithms. The present invention describes a method andapparatus to measure blood flow in real-time by using a DSP for digitalprocessing of the power spectra of blood flow signal.

In digital form, the above weighting function can be written as:${\omega \quad \text{weighting:Flux=}{\int_{\omega_{1}}^{\omega_{2}}{\omega \quad {P(\omega)}{\omega}}}} = {{\sum\limits_{n}}_{1}^{n_{2}}{n\quad {P(n)}}}$${\omega^{2}\quad \text{weighting:Flux=}{\int_{\omega_{1}}^{\omega_{2}}{\omega^{2}\quad {P(\omega)}{\omega}}}} = {{\sum\limits_{n}}_{1}^{n_{2}}{n^{2}\quad {P(n)}}}$

and noise subtracted and normalised forms Flux_(sn) are

ω weighting:Flux_(sn)=(ΣnP(n)−Noise)/DC ²

(ω² weighting:Flux_(sn)=(Σn ² P(n)−Noise)/DC ²

Noise=SN×DC+DN

where n₁ and n₂ are lower and upper limits of frequency components inthe computation, P(n) is the power spectra density of the nth frequencycomponent, Noise is the system noise which includes dark noise (DN) andDC proportional shot noise (SN). DC is a measurement of the intensity ofthe collected scattered light.

A detailed implementation of the above algorithms is illustrated in FIG.2. As an example, the Doppler signal (AC) is sampled at 32 KHz and1024-point FFT is used. When 1024 points of data are sampled, data ismultiplied by a twiddle cosine window table to reduce artefactualspectral content resulting from discontinuities at the start and endpoints of the sampled wave form, and then is converted into frequencydomain by FFT, and the weighting function, noise subtraction,normalisation and smoothing are performed by the DSP. After the FFTtransformation of the 1024 points of data is completed, the DSP startsto sample the next 1024 points of Doppler signals so that a higher datarate can be achieved. The employed DSP system enables sampling and fluxcalculation to be performed in approximately 33 ms, so that a data rateof 30 Hz is possible.

FIG. 3 illustrates the use of a DSP in a multichannel system. Thisexample is a 16 channel laser Doppler system which can achieve 10 Hzdata rates for 256 point FFT's.

By the means of digital spectra processing of the Doppler signaldescribed, a continuous blood flow output is produced. It is apparentthat both ω and ω² weighting or other spectra analysis algorithms can beeasily implemented without significantly altering the concept involved.Also, different frequency ranges of the Doppler signal can be analysedseparately by choosing the lower and upper limits of frequencycomponents. For example, if it is known that blood flow signal for aparticular application is toward high frequency band, low frequencycomponents can be ignored by increasing the lower limit n₁ to produceless noise flow output. Another example is to calculate the ratio offlow from high frequency band and low frequency band using the presentinvention apparatus. Furthermore, other parameters, such as averagevelocity of the blood flow, concentration can be calculated in a similarway.

The average Doppler frequency shift <ω> of the light scattered frommoving red blood cells is directly proportional to the average speed ofthese cells. ⟨ω⟩ = ∫_(ω₁)^(ω₂)ω  P(ω)ω/∫_(ω₁)^(ω₂)P(ω)ω

Red blood cell (rbc) concentration is proportional to the integratedpower spectral density for low concentration (less than 0.5%) i.e.rbc  concentration ∝ ∫_(ω₁)^(ω₂)P(ω)ω

MOVEMENT ARTEFACT

Movement artefact can be a major problem for the clinical use of a laserDoppler flow monitor based on fibre-optic transducers. Clinical studiesoften reveal changes in the blood flow signal which are unrelated toactual physiological changes in blood flow and are usually produced bymovement of the optical fibres. Also when the means of tissueirradiation is via a laser beam relative movements of beam and tissuesurface can produce noise components similar to those generated by fibremovement. In many applications of laser Doppler flow monitors, it ispossible to ensure that a subject remains still during the measurement.However, this is not feasible in some conditions, such as cerebralperfusion monitoring, intrapartum monitoring or monitoring a baby.Although there has been a recent trend by many LDF equipmentmanufacturers to move to small diameter optical fibres in order toreduce movement artefact, the problem still persists. Thecommercially-available laser Doppler flow monitor known as Perimed PF3employs an analogue circuit to reject movement artefact simply based onslope rate of the blood flow signal. If the rate of change of slope inthe blood flow signal exceeds the likely physiological change, theoutput is switched off until such abrupt change has discontinued.However, when the fibre movements are small, the system finds itdifficult to distinguish movement artefact from genuine changes in bloodflow. Continuous fibre movement of large enough amplitude to trigger therejection filter leads to a bizarre situation in which the blood flowoutput is unavailable during most of the recording period.

The present invention comprises a means of frequency analysis to detectmovement artefact in real-time and a means to provide a continuous bloodflow output even during fibre movement.

Fibre movement is known to produce an increase in the Doppler beatfrequency spectrum and is generally considered to be the result of thechanging modal interference pattern. For the purpose of identifying thefrequencies influenced by the fibre movement, a set of experiments hasbeen conducted in which the fibre was placed near a mechanical armcontrolled by a DC motor and movement artefact was produced by drivingthe mechanical arm forwards and backwards to hit the fibre. FIG. 4 showsthe power spectra obtained from skin with (a) and without (b) fibremovement. It can be seen that the effect of fibre movement was mainlyconfined to the lower part of the beat frequency spectrum associatedwith the blood flow signal, particularly below 3 KHz, and has lessinfluence on the higher frequency range related to fast blood cells.Therefore, by calculating the change of the spectral power in a lowfrequency band (e.g. 20 Hz-3 KHz), it is possible to detect movementartefact which causes a sudden increase in the power density on thelower frequency range, while blood flow increases which mainly changesover higher frequency range will not be mis-detected as noise. With theuse of DSP and the fast Fourier transformation, noise reductionalgorithms can be easily implemented without any change to the hardware.

In an example of a noise reduction algorithm illustrated here, twoparameters are calculated together with the blood flux. One is thecurrent value of the ω weighted power density (LP) over the lowfrequency band (e.g. 20 Hz-3 KHz) and the other is the averaged ωweighted power density over this frequency band (LPA).${LP} = {\sum\limits_{1}^{N}{{n \cdot {P(n)}}\quad {and}}}$LPA = LPAold + (LP − LPAold) × α

where N is the number of Fourier components in the range 0 to 3 KHz andLPAold is the value of LPA previous to the calculation of LPA.

This equation describes low pass filtering of LP, to produce an averagedvalue, where α is a parameter inversely proportional to the timeconstant of the filter, i.e.$\alpha = {\frac{1}{f_{s}} \times \frac{1}{TC}}$

where f_(s) is the signal sample rate and TC is the time constant.

For example $\begin{matrix}{{{For}\quad {example}\quad f_{s}} = {{30{Hz}\quad {and}\quad {TC}}\quad = \quad {1.0s}}} \\{\alpha = 0.033}\end{matrix}$

LP is compared with LPA to determine whether or not LP has a significantnoise content.

The time constant (TC) can be preset, or in some cases calculatedautomatically from spectra measured with and without noise induced, tosuit the characteristics of the monitored blood flow signal. A long(TC), e.g. 1.0 s, will result in a relatively stable value for LPA sothat sensitivity to a noisy LP value is high; however if the timeconstant is too long the noise filter could be triggered by a pulsatileflux signal which has its origins in a physiological change in additionto noise triggering. A fixed large TC is therefore suitable only whenslow changes in blood flow are to be recorded, for example monitoringtrends over minutes or hours. For monitoring fast changing flux changese.g. changes associated with the cardiac cycle a short time constante.g. TC=0.1 s is appropriate.

Triggering of the filter is set to occur when the LP exceeds LPA scaledby an appropriate coefficient i.e. triggering if LP>s×LPA

Typically s has a value between 1.5 and 2.5.

A lower value could result in filtering out of a physiological changeand a higher value may not filter out a noise signal.

For this present example filtering can be done in two ways. During theperiod for which LP>s×LPA either the noisy signal is replaced byrelatively noise free data recorded in a period immediately prior to thenoisy period (noise replacement filtering) or the noise is reduced bycalculating the flux with the noisy LP value replaced by LPA. In thislatter case the unnormalised flux is calculated as:

Flux=LPA+HP

where HP is the ω weighted power spectral density for the high frequencyband e.g. 3 KHz to 15 KHz.${HP} = {\sum\limits_{N}^{M}{n\quad {P(n)}}}$

where N corresponds to 3 KHz and M to 15 KHz.

After movement artefacts are detected, in noise replacement filtering, ashort period (for example 1 sec) “eye close” scheme is introduced, whichis based on the fact that modal pattern fluctuations produced by asudden short lived fibre movement will normally die out after a periodof 1 second or less. During this period no further noise detection isperformed and blood flow signal contaminated by the movement artefactwill be replaced by the previous 1 sec of data which has been stored inthe DSP. If the signal is still judged to be noisy at the end of the 1second period, noise is again replaced by the earlier low noise signal.If pulsatile blood flow signal is monitored, the length of the “eyeclose” period can be changed according to the latest pulse rate, so oneor more cycles of blood flow data can be used to replace the noisecontaminated signal in order to retain the pulsatile nature. A measuringapparatus constructed in the afore described manner in accordance withthe invention was evaluated. FIG. 5(a) illustrates a noisy signal (noiseproduced by fibre movement) measured from the Brownian motion ofmicrospheres in water and FIG. 5(b) shows the recorded signal when thenoise replacement filter is applied with a time constant of 0.5 s and ascaling coefficient of 2.0.

FIG. 6(a) illustrates a noisy signal measured from a finger tip. Theflux is pulsatile, with a repetition rate equal to the volunteer's pulserate.

FIG. 6(b) shows the recorded signal when the filter is applied with atime constant of 0.1 s, a scaling coefficient of 2.0, and noisy signalsreplaced by low noise pulsatile signals.

It is apparent that various changes and modifications, such as lookingat different frequency ranges, employing a sophisticated adaptivethreshold detection algorithm for noise detection, using averaged lowfrequency power spectra to replace noise contaminated signal over lowfrequency range can also be implemented.

Frequency ranges other than the 20 Hz-3 KHz can be used depending on theoptic fibre type used, because in general the smaller the core diameterthe smaller the frequency range of movement artefact noise signals. Forexample, a lower frequency band 20 Hz-1 KHz used with 50 micron corediameter fibre enables good discrimination between signal and noise.

For a measurement protocol where a very rapid increase in flux ispredicted, (e.g. the release of a pressure cuff occluding or partiallyoccluding blood flow into a limb) the algorithm applied in the filterexample described will not be able to discriminate between movementartefact noise and signal. If this is so provision can be made to switchoff the filter immediately prior to pressure release to enable the rapidflux change to be recorded.

The timing for the pressure release and the switching off of the filtercan be pre-programmed to coincide if a suitable control program isavailable for the laser Doppler monitor or the filter can be turned offusing a trigger signal from a pressure transducer.

The present invention provides an alternative to switching off thefilter by applying algorithms which enable discrimination between noisesignals and rapidly varying flux signals which have their origins in aphysiological change. This requires both high (HPA) and low (LPA)frequency average ω weighted power density parameters to be calculatedand comparisons to be made with their respective current values (HP) and(LP).

HPA=HPAold+(HP−HPAold)×α

the high frequency band equivalent of the LPA equation.

For a pressure cuff release resulting in reactive hyperaemia (generallya large increase in blood perfusion) both LP and HP will increasesignificantly whereas if the increase was due to noise only LP willsignificantly increase.

The noise filter is set to turn ON if:

LP>s×LPA

and

HP<g HPA

where g is a scaling coefficient value typically 1.5 to 2.5. Thiscondition, a large increase in LP and a small or zero increase in HP, ischaracteristic of movement artefact signal noise.

The noise filter will not turn ON and hence the flux change will berecorded if:

HP>g HPA

The scaling coefficient can be automatically set by recordingphotocurrent spectra for fibre noise and for reactive hyperaemia, thoughbecause of possible non standard responses provision is made to switchoff the noise filter manually or by using a trigger signal from apressure transducer.

The relative changes in LP and HP can also be used in a direct way todiscriminate between noisy and relatively noise free signals. The normalrange LP/HP ratios from noise free signals can be recorded and used asreference levels so that abnormal LP/HP ratios are used to trigger ONthe noise filter.

For blood perfusion recordings of trends over very long periods e.g.several hours very low data rates are appropriate for example 1 datapoint for each 10 second period. As noise always results in apparentsignal increase and as movement artefact generated noise is seldomcontinuous an effective method of noise reduction in the recorded signalis to record the minimum detected signal level for each 10 secondmeasurement period. A short time constant is used for the fluxcalculations to ensure that the effect of large noise signals in theflux calculation is short lived. During any 10 second period there isthen a high probability of a signal sample taken in noise free (i.e.movement artefact noise free) conditions.

This filter has the advantage of simplicity and indeed does not requirefrequency analysis of the photocurrent for its implementation. Itsdisadvantage is that it is associated with a low data recording rate sothat detail of events occurring within a sampling period is notrecorded.

Filters which rely on the information derived from frequency analysis ofthe photocurrent spectra not only filter out a significant proportion ofthe noise signals but also enable high data recording rates to beimplemented. Noise is filtered while at the same time information onchanges in blood perfusion associated with the cardiac cycle,respiration, thermo regulation and vasomotion are recorded.

We claim:
 1. An apparatus for measuring blood in tissue comprising: amonochromatic laser light source; means for irradiating a section of thetissue with the monochromatic light from the light source; means forcollecting light scattered from the irradiated section; a photodetectorfox detecting the collected scattered light; means for processingelectrical output signals from the photodetector; means for calculatinga power spectrum of photocurrents generated in the detection of laserlight scattered from static tissue and Doppler broadened laser lightscattered from moving blood cells; means for calculating and recordingthe average Doppler frequency shift; means for calculating and recordingthe red blood cell concentration; means for measuring and recording theintensity of the detected scattered light; means for calculating andrecording the blood perfusion (flux); means for filtering movementartefact noise by measuring changes in the photocurrent power spectrumfor a low frequency band; means for displaying the blood perfusionmeasured parameters; means to judge whether a change is due to movementartefact noise by comparison of current low frequency spectral powerwith averaged low frequency power or by comparison of current frequencyweighted low frequency spectral power (LP) with the averaged frequency(ω) weighted low frequency spectral power(LPA); means to calculate ablood perfusion (flux) value in the presence of detected movementartefact noise by summing the averaged frequency (ω) weighted lowfrequency spectral power (LPA) with frequency weighted high frequencyspectral power (HP), where: Flux=Instrument Constant×(LPA+HP); means tocalculate a blood perfusion value in the absence of movement artefactnoise by summing the current frequency weighted low frequency spectralpower (LP) and the frequency weighted high frequency spectral power(HP), where: Flux=Instrument Constant×(LP+HP); and means to calculate ablood perfusion value in the presence of movement artefact noisecomprising a noise replacement filter whereby during a noise affectedperiod the noisy signal is replaced by noise free data recorded in aperiod immediately prior to the noisy period.
 2. An apparatus accordingto claim 1, wherein the means for irradiating a section of tissue withmonochromatic laser light is via an optic fibre and the means to collectlight scattered from the tissue for photodetection is via one or moreoptic fibres.
 3. An apparatus according to claim 1, wherein the meansfor irradiating a section of tissue with monochromatic laser light iswith a laser beam.
 4. An apparatus according to claim 1, wherein thesignal frequency analysis, calculations of blood flow parameters andsignal filtering to reduce the affects of movement artefact on themeasured blood perfusion, red blood cell concentration and averagedDoppler frequency shift are carried out with the aid of fast large scaledigital signal processing integrated circuits to enable real timeprocessing and display.
 5. An apparatus for measuring blood in tissuecomprising: a monochromatic laser light source; means for irradiating asection of the tissue with the monochromatic light from the lightsource; means for collecting light scattered from the irradiatedsection; a photodetector for detecting the collected scattered light;means for processing electrical output signals from the photodetector;means for calculating a power spectrum of photocurrents generated in thedetection of laser light scattered from static tissue and Dopplerbroadened laser light scattered from moving blood cells; means forcalculating and recording the average Doppler frequency shift; means forcalculating and recording the red blood cell concentration; means formeasuring and recording the intensity of the detected scattered light;means for calculating and recording the blood perfusion (flux); meansfor filtering movement artefact noise by measuring changes in thephotocurrent power spectra for both low and high frequency bands; meansfor displaying the blood perfusion measured paraeters; means tocalculate frequency (ω) weighted averaged spectral power for a lowfrequency band (LPA) and for a high frequency band (HPA); means to judgewhether changes are due to movement artefact noise by comparison ofcurrent frequency weighted low frequency spectral power (LP) with theaveraged frequency weighted low frequency spectral power (LPA) andcomparison of current frequency weighted high frequency spectral power(HP) with the averaged frequency weighted high frequency spectral power(HPA); and means to calculate a blood perfusion (flux) value in thepresence of movement artefact noise detected by said comparison bysumming the averaged frequency weighted low frequency spectral power(LPA) with the current frequency weighted high frequency spectral power(HP).
 6. An apparatus according to claim 5, wherein the means forirradiating a section of tissue with monochromatic laser light is via anoptic fibre and the means to collect light scattered from the tissue forphotodetection is via one or more optic fibres.
 7. An apparatusaccording to claim 5, wherein the means for irradiating a section oftissue with monochromatic laser light is with a laser beam.
 8. Anapparatus according to claim 5, wherein the signal frequency analysis,calculations of blood flow parameters and signal filtering to reduce theaffects of movement artefact on the measured blood perfusion, red bloodcell concentration and averaged Doppler frequency shift are carried outwith the aid of fast large scale digital signal processing integratedcircuits to enable real time processing and display.
 9. An apparatus formeasuring blood in tissue comprising: a monochromatic laser lightsource; means for irradiating a section of the tissue with themonochromatic light from the light source; means for collecting lightscattered from the irradiated section; a photodetector for detecting thecollected scattered light; means for processing electrical outputsignals from the photodetector; means for calculating the power spectrumof photocurrents generated in the detection of laser light scatteredfrom static tissue and Doppler broadened laser light scattered frommoving blood cells; means for calculating and recording the averageDoppler frequency shift; means for calculating and recording the redblood cell concentration; means for measuring and recording theintensity of the detected scattered light; means for calculating andrecording the blood perfusion (flux); means for filtering movementartefact noise by sampling blood perfusion values at a rate comparableto the rate at which blood perfusion values are calculated; means fordisplaying the blood perfusion measured parameters; means to record theminimum perfusion value taken in a measurement period of long durationcompared to the calculation period of the blood perfusion values and ofshort duration compared to the period for which the trend is to berecorded; means to record minimum values for successive search periods;and means to display the blood perfusion minimum values for longmeasurement periods to produce trend records substantially free ofmovement artefact noise.
 10. An apparatus according to claim 9, whereinthe means for irradiating a section of tissue with monochromatic laserlight is via an optic fibre and the means to collect light scatteredfrom the tissue for photodetection is via one or more optic fibres. 11.An apparatus according to claim 9, wherein the means for irradiating asection of tissue with monochromatic laser light is with a laser beam.12. An apparatus according to claim 9, wherein the signal frequencyanalysis, calculations of blood flow parameters and signal filtering toreduce the affects of movement artefact on the measured blood perfusion,red blood cell concentration and averaged Doppler frequency shift arecarried out with the aid of fast large scale digital signal processingintegrated circuits to enable real time processing and display.